Caption: Viagra impotence pills. Bottle and a 100mg Viagra pill (sildenafil citrate) to treat male impotence. Viagra is the first oral pill for impo- tence. Unlike other treatments that directly cause penile erections, Viagra affects the response to sexual stimulation. Viagra enhances the smooth muscle relaxant effects of nitric oxide, a substance normally released in response to sexual stimulation. Smooth muscle relaxation allows blood to enter the penis leading to an erection. Viagra taken by women may also affect their sexual response. The pills are taken one hour before sexual activity; 25mg & 50mg pills are available.